This protocol tests the efficacy of exogenous melatonin for shifting the timing of the internal biological clock in a simulated of nightshift work paradigm. Melatonin (0.5 mg and 10 mg) and placebo will be tested in subjects who shift their sleep-wake cycle in either an advance or delay direction. In order to rule out an effect of bright light on circadian rhythms, the study will be conducted in controlled, dim-light conditions.